An antenna is said to be a small antenna (a miniature antenna) when it can be fitted in a space which is small compared to the operating wavelength. More precisely, the radiansphere is taken as the reference for classifying an antenna as being small. The radiansphere is an imaginary sphere of radius equal to the operating wavelength divided by two times π; an antenna is said to be small in terms of the wavelength when it can be fitted inside said radiansphere.
The fundamental limits on small antennas where theoretically established by H. Wheeler and L. J. Chu in the middle 1940's. They basically stated that a small antenna has a high quality factor (Q) because of the large reactive energy stored in the antenna vicinity compared to the radiated power. Such a high quality factor yields a narrow bandwidth; in fact, the fundamental limit derived in such theory imposes a maximum bandwidth given a specific size of an small antenna. Other characteristics of a small antenna are its small radiating resistance and its low efficiency.
The development of innovative structures that can efficiently radiate from a small space has an enormous commercial interest, especially in the environment of mobile communication devices (cellular telephony, cellular pagers, portable computers and data handlers, to name a few examples), where the size and weight of the portable equipments need to be small. According to R. C. Hansen (R. C. Hansen, “Fundamental Limitations on Antennas,” Proc.IEEE, vol.69, no.2, February 1981), the performance of a small antenna depends on its ability to efficiently use the small available space inside the imaginary radiansphere surrounding the antenna. In the present invention, a novel set of geometries named ring-like space-filling surfaces (RSFS) are introduced for the design and construction of small antennas that improves the performance of other classical microstrip patch antennas described in the prior art.
A general configuration for microstrip antennas (also known as microstrip patch antenans) is well known for those skilled in the art and can be found for instance in (D. Pozar, “Microstrip Antennas: The Analysis and Design of Microstrip Antennas and Arrays”. IEEE Press, Piscataway, N.J. 08855-1331). The advantages such antennas compared to other antenna configurations are its low, flat profile (such as the antenna can be conformally adapted to the surface of a vehicle, for instance), its convenient fabrication technique (an arbitrarily shaped patch can be printed over virtually any printed circuit board substrate), and low cost. A major draw-back of this kind of antennas is its narrow bandwidth, which is further reduced when the antenna size is smaller than a half-wavelength. A common technique for enlarging the bandwith of microstrip antennas is by means of a parasitic patch (a second patch placed on top of the microstrip antenna with no feeding mechanism except for the proximity coupling with the active patch) which enhances the radiation mechanism (a description of the parasitic patch technique can be found in J. F. Zurcher and F. E. Gardiol, “Broadband Patch Antennas”, Artech House 1995.). A common disadvantage for such an stacked patch configuration is the size of the whole structure.